The thing is that you haven’t explained why the messages are being sent. Is it the floor of the stock exchange or a raging battle? Time of day/night matters, too.

There’s a tendency in fantasy books to over-complicate things for the sake of world building. People have been arguing about the lack of safety rails on the Death Star for decades.

Unless it’s vital to the plot I’d say stick with a tried and true tech.

That said, you could throw somethign the size of a coconut pretty far with a sling or some form of catapault, and adding a bright colored ribbon would look good and you could give the colors meanings; like white for surrender or blood red for an individual challenge.